Wednesday, July 20, 2011

Accessing the data center from the cloud with OpenVPN

This post was inspired by a recent exercise I went through at the prompting of my colleague Dan Mesh. The goal was to have Amazon EC2 instances connect securely to servers at a data center using OpenVPN.

In this scenario, we have a server within the data center running OpenVPN in server mode. The server has a publicly accessible IP (via a firewall NAT) with port 1194 exposed via UDP. Cloud instances which run OpenVPN in client mode are connecting to the server, get a route pushed to them to an internal network within the data center, and are then able to access servers on that internal network over a VPN tunnel.

Here are some concrete details about the network topology that I'm going to discuss.

Server A at the data center has an internal IP address of 10.10.10.10 and is part of the internal network 10.10.10.0/24. There is a NAT on the firewall mapping external IP X.Y.Z.W to the internal IP of server A. There is also a rule that allows UDP traffic on port 1194 to X.Y.Z.W.

I have an EC2 instance from which I want to reach server B on the internal data center network, with IP 10.10.10.20.

Install and configure OpenVPN on server A

Since server A is running Ubuntu (10.04 to be exact), I used this very good guide, with an important exception: I didn't want to configure the server in bridging mode, I preferred the simpler tunneling mode. In bridging mode, the internal network which server A is part of (10.10.10.0/24 in my case) is directly exposed to OpenVPN clients. In tunneling mode, there is a tunnel created between clients and server A on a separated dedicated network. I preferred the tunneling option because it doesn't require any modifications to the network setup of server A (no bridging interface required), and because it provides better security for my requirements (I can target individual servers on the internal network and configure them to be accessed via VPN). YMMV of course.

For the initial installation and key creation for OpenVPN, I followed the guide. When it came to configuring the OpenVPN server, I created these entries in /etc/openvpn/server.conf:

The first directive specifies that the OpenVPN tunnel will be established on a new 172.16.0.0/24 network. The server will get the IP 172.16.0.1, while OpenVPN clients that connect to the server will get 172.16.0.6 etc.

The second directive pushes a static route to the internal data center network 10.10.10.0/24 to all connected OpenVPN clients. This way each client will know how to get to machines on that internal network, without the need to create static routes manually on the client.

You should also see routing information related to both the tunneling network 172.16.0.0/24 and to the internal data center network 10.10.10.0/0 (which was pushed from the server):

# netstat -rn

Kernel IP routing table

Destination Gateway Genmask Flags MSS Window irtt Iface

172.16.0.5 0.0.0.0 255.255.255.255 UH 0 0 0 tun0

172.16.0.1 172.16.0.5 255.255.255.255 UGH 0 0 0 tun0

10.0.10.0 172.16.0.5 255.255.255.0 UG 0 0 0 tun0

....etc

At this point, the client and server A should be able to ping each other on their 172.16 IP addresses. From the client you should be able to ping server A's IP 172.16.0.1, and from server A you should be able to ping the client's IP 172.16.0.6.

Create static route to tunneling network on server B and enable IP forwarding on server A

Remember that the goal was for the client to access server B on the internal data center network, with IP address 10.10.10.20. For this to happen, I needed to add a static route on server B to the tunneling network 172.16.0.0/24, with server A's IP 10.10.10.10 as the gateway:

# route add -net 172.16.0.0/24 gw 10.10.10.10

The final piece of the puzzle is to allow server A to act as a router at this point, by enabling IP forwarding (which is disabled by default). So on server A I did:

# sysctl -w net.ipv4.ip_forward=1

# echo "net.ipv4.ip_forward=1" >> /etc/sysctl.conf

At this point, I was able to access server B from the client by using server B's 10.10.10.20 IP address.

We've just started to experiment with this setup, so I'm not yet sure if it's production ready. I wanted to jot down these things though because they weren't necessarily obvious, despite some decent blog posts and OpenVPN documentation. Hopefully they'll help somebody else out there too.

I don't know how others have done it but thought I'd leave my thoughts here seeing as I've been using this style set-up in production From Ec2 to Datacenter for about 6 months now. Each server in EC2 we run has openVPN client automatically configured and running, when its started it connects up to the openVPN server and everything has been working great. The more routes/IPs available at the OpenVPN server end the better just for protection of failing links into your datacenter really.

The connection has been really reliable for each server and not really had any problems with the set-up at all (of which its a critical part of our application). One thing I'll add is the following that should be added to your configs...

Server Sidekeepalive 2 6 #This is so that the client side if connection failure does occur, should work out the connection has failed sooner than usual and then start recovery and try re-connecting. I found these values to work pretty well much lower and I noticed traffic speed through the tunnel was badly impacted.

Client Sideconnect-retry 1 #This is again for speed of re-negotiating connections on VPN failure. Though i can say this happens pretty much never and is more helpful when restarting the OpenVPN server so that clients re-connect quickly.